Re: [Xen-devel] [PATCH v2 1/4] x86/compat: Test whether guest has 32b shinfo instead of being a PV 32b domain

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.07.15 at 19:13, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 07/07/2015 12:15 PM, Jan Beulich wrote:
>>>>> On 07.07.15 at 17:46, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 07/07/2015 05:11 AM, Jan Beulich wrote:
>>>>>>> On 29.06.15 at 22:21, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> @@ -737,7 +737,7 @@ int arch_set_info_guest(
>>>>>    
>>>>>        /* The context is a compat-mode one if the target domain is 
>>>>> compat-mode;
>>>>>         * we expect the tools to DTRT even in compat-mode callers. */
>>>>> -    compat = is_pv_32on64_domain(d);
>>>>> +    compat = has_32bit_shinfo(d);
>>>> Furthermore, looking at uses like this, tying such decisions to the
>>>> shared info layout looks kind of odd. I think for documentation
>>>> purposes we may need a differently named alias.
>>> Yes, it does look odd, which is why I was asking in another thread about
>>> having another field in domain structure (well, I was asking about
>>> replacing has_32bit_shinfo but I think I can see now that wouldn't be
>>> right).
>>>
>>> Are you suggesting a new macro, e.g.
>>> #define is_32b_mode(d)    ((d)->arch.has_32bit_shinfo)
>>>
>>> or would it better to add new field? Or get_mode() hvm op, similar to
>>> set_mode(), which can look, say, at EFER?
>> If looking at EFER (plus perhaps CS) is right in all the cases you
>> care about, then yes. And remember we already have
>> hvm_guest_x86_mode().
> 
> Can't use hvm_guest_x86_mode(), it asserts on 'v != current'. But adding 
> new op just because of that seems to be an overkill since it would 
> essentially do what .guest_x86_mode() does. How about 
> hvm_guest_x86_mode_unsafe() (with a better name) and wrap 
> hvm_guest_x86_mode() with the ASSERT around it?

svm_guest_x86_mode() doesn't depend on v == current, but
vmx_guest_x86_mode() would first need to be made safe (or
get an "unsafe" sibling implementation). With that, the ASSERT()
could then check for current or non-running vCPU.

>>>>> --- a/xen/common/domctl.c
>>>>> +++ b/xen/common/domctl.c
>>>>> @@ -496,7 +496,7 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) 
> u_domctl)
>>>>>                break;
>>>>>    
>>>>>    #ifdef CONFIG_COMPAT
>>>>> -        if ( !is_pv_32on64_domain(d) )
>>>>> +        if ( !has_32bit_shinfo(d) )
>>>>>                ret = copy_from_guest(c.nat, op->u.vcpucontext.ctxt, 1);
>>>>>            else
>>>>>                ret = copy_from_guest(c.cmp,
>>>>> @@ -902,7 +902,7 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) 
> u_domctl)
>>>>>            vcpu_unpause(v);
>>>>>    
>>>>>    #ifdef CONFIG_COMPAT
>>>>> -        if ( !is_pv_32on64_domain(d) )
>>>>> +        if ( !has_32bit_shinfo(d) )
>>>>>                ret = copy_to_guest(op->u.vcpucontext.ctxt, c.nat, 1);
>>>>>            else
>>>>>                ret = 
>>>>> copy_to_guest(guest_handle_cast(op->u.vcpucontext.ctxt,
>>>> Where is it written down what format 32-bit PVH guests' vCPU
>>>> contexts get passed in? It would seem to me that it would be
>>>> rather more natural for them to use the 64-bit layout. Or else
>>>> how do you intend to suppress them being able to enter 64-bit
>>>> mode?
>>> So why do we use the 'else' clause for 32b PV guests when they also use
>>> the same vcpu_guest_context_x86_32_t in libxc/xc_dom_x86.c:vcpu_x86_32()?
>> 32bit PV guests use the if() branch afaict (as they use the 32-bit
>> shared info layout).
> 
> No, they use the 'else' part, I just confirmed it. 'd' in 
> is_pv_32on64_domain() is domain for which domctl is being called, not 
> domain that is making the call (which is what I suspect the original 
> intent was).

Oh, yes, of course they do - how did I overlook the "!" ? Yet
that doesn't help me understand the question: Isn't it obvious
that if libxc expects vcpu_guest_context_x86_32_t, then the
hypervisor also needs to supply that one (and not the 64-bit
counterpart)? Or are you asking why the format matches the
subject domain's word width, not the calling domain's? This has
historical reasons: A 32-bit domain saved on a 64-bit hypervisor
needed to be restorable by a 32-bit hypervisor when that still
existed. This could likely be changed nowadays; ARM and the
HVM case must be dealt with in the tools somehow anyway.

Jan

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